Mitochondria – Powerhouse of Cells
You would be surprised to know that the phrase does present one of the major components of the cells that are what makes our body. Mitochondria is indeed the powerhouse of human cells, and these were discovered by Albert von Kolliker in 1857 and were later termed as ‘bioblasts’ by Richard Altman in 1886. Later, somewhat twelve years later these organelles were renamed mitochondria by Carl Benda.
Nowadays, the mighty mitochondria play a great role in providing energy to the cell present in the human body. Hence, they help to maintain the cellular metabolism and even regulate cell survival and death. Frankly, these organelles are responsible for the conversion of food molecules and oxygen into adenosine triphosphate (ATP).
The ATP would be used in providing energy for the cell. Surprisingly, due to the central role that they play in the cell, certain dysfunctions present in mitochondria have been linked to many diseases. Recently, the defects presented in mitochondria are linked to the process of neurodegeneration. This is the progressive death or loss in function of neurons, the cells that make up our brains and nervous system.
Such kind of damage to distinct subsets of neurons is the root cause of diseases such as Alzheimer’s, Parkinson’s, Huntington’s, and Amyotrophic Lateral Sclerosis (ALS, or Lou Gherig’s disease). That means whether mitochondrial impairment would lead to neurodegeneration in such diseases or is merely a consequence of it remains unclear. However, there is evidence that clearly shows that these organelles play a strong role in neurodegeneration is growing.
Definition of Mitochondria
Mitochondria are often called the powerhouses of the cell. Basically, they are tiny organelles that float around in almost all kinds of cells present in your body. Keep in mind that red blood cells are the only kind that won’t have any.
The mitochondria make up around 25% of the volume of any given cell and that too except for red blood cells.
Mitochondria unlock the energy stored in the foods we eat and make it available to power our bodies. In fact, the vast majority of the chemical energy cells need to survive is produced by mitochondria. Here these so-called organelles do perform other major functions. Plus, they play a great role in cell survival and even detoxification.
That means the mitochondria break down and even recycles the waste products. Hence, they end up being less harmful. Also, they show how the cells would be responding to injury and infections. Due to them these cells work properly and are healthy.
During evolution, these essential organelles used to be independent bacterium that got engulfed by another cell. They started working together, and over time the resulting mitochondria play a key role in our bodies.
That’s why mitochondria are semi-autonomous and have their own DNA, which is in the circular shape just as in bacteria. Furthermore, they even have their own machinery to produce several mitochondrial proteins. That means even if they boast a degree of independence, they constantly interact with the rest of the cell. This communication is essential for cellular homeostasis in health and disease.
Working of Mitochondria
Mitochondria are created from the process called mitochondrial biogenesis. Such types of organelles would get interconnected in complex and beautiful networks. That means part of these structures fragment and create smaller mitochondria, through the procedure called mitochondrial fission.
On the other hand, new mitochondria would fuse together and increase the size and complexity of the network. Such fusion or fission dynamics maintain a healthy organelle and adapt them to different cellular and metabolic conditions.
Surprisingly, mitochondria have a double membrane. The inner part is folded to increase surface area. This is where many steps of aerobic cellular respiration take place.
Cellular respiration is the process that turns the food we eat into fuel for the cell’s activities. It produces the majority of the body’s main source of energy, in the form of ATP molecules (short for adenosine triphosphate). On the other hand, many cellular processes depend on ATP, two examples being that it powers muscle contractions and cellular division, among other things.
Structure of Mitochondria
- Size ranges from 0.5 to 1.0 micrometres in diameter.
- Mitochondria are found in both plant and animal cells.
- The structure consists of an outer membrane, an inner membrane, and a jelly-like material known as a matrix.
- They are double-membraned and rod-shaped in structure.
Outer Membrane
From this membrane, small molecules could pass freely. It covers the surface of the mitochondria. It consists of many special proteins called porins which form channels that allow proteins to cross. Also, the outer membrane of mitochondria can host many enzymes with a wide variety of functions. Here the intermembrane is the space between the inner and outer membranes.
Inner Membrane
This is even made up of major proteins having many roles. As it does not consist of porins and so, it is impermeable to most molecules. Molecules can only cross the inner membrane in special membrane transporters. In the inner membrane, only ATP is generated.
Matrix
The space which is present in the inner membrane is known as the matrix. It contains various enzymes, which are important in the production of ATP. Here, mitochondrial DNA is housed.
Different types of cells have different numbers of mitochondria. Like mature red blood cells have none, whereas liver cells contain more than 2,000. High-demanding energy cells have greater numbers of mitochondria.
Hence, we can say that mitochondria are freely permeable to ions, nutrient molecules, energy molecules like the ADP, and ATP molecules.
Cristae
There can be folds present in the inner membrane. These are called cristae. Frankly, because of the surface area present in the membrane, it would increase. That means the space present for the chemical reaction even increases.
Major Functions of Mitochondria
- Promotes the growth of new cells and cell multiplication
- Regulates the metabolic activity of the cell
- Plays an important role in apoptosis or programmed cell death
- Helps in detoxifying ammonia in the liver cells
- Helps in maintaining an adequate concentration of calcium ions within the compartments of the cell
- It is also involved in various cellular activities like cellular differentiation, cell signalling, cell senescence, controlling the cell cycle, and in cell growth.
- Responsible for building certain parts of the blood and various hormones like testosterone and estrogen.
What Would Occur if Mitochondria Got Damaged and Malfunctioned?
Over time, there can be a natural decline in mitochondria quality and activity, which is associated with normal aging. Here the mitochondria are also sensitive to nutrient deficiencies, environmental toxins, and oxidative damage. When it comes to a healthy cell, damaged or old mitochondria that don’t function at their best can be recycled through a process called mitophagy. Once digested, their parts are recycled.
If mitochondria get damaged or malfunctioning and normal cell processes to fix them can’t act, then the whole cellular health gets negatively impacted, and consequently, our overall health suffers. For example, the reduced mitochondrial function can cause our muscles to become weaker, our cognitive function to decline, and our immune system to work sub-optimally.
Mitochondrial dysfunction is a hallmark of aging and contributes to several age-associated issues, such as muscle weakness. This can be based on and even develop into sarcopenia, a severe form of decline in skeletal muscle function) and cognitive impairments.
Hence, those mitochondrial defects that prolong over time and/or become more severe can play an important role in the progression of various diseases linked to organs having a high density of mitochondria for their energy needs, such as neurodegenerative disorders like Alzheimer’s Disease, Parkinson’s Disease, cardiovascular diseases, diabetes, autoimmune diseases, etc.
Mitochondria Are Necessary for Our Human Body
As clearly stated above mitochondria are referred to as the powerhouse of the cell or energy factories because they help in extracting energy from food via cellular respiration. The energy is released in the form of adenosine triphosphate (ATP). It is also called the energy currency of the cell.
The Powerhouse Function of Mitochondria
In the digestion process, food breaks down and products from the digestion of food find their way into the cell then a series of chemical reactions occur in the cytoplasm. This permits some of the energy locked up in these products to be released and incorporated into the universal energy supplier in cells called ATP.
From this process, remaining molecular fragments then enter the mitochondria, and there they are finally converted into carbon dioxide and water. In these fragments, the energy locked up is incorporated into more ATP.
Causing Mitochondrial Disease
Most mitochondrial diseases are due to mutations in nuclear DNA that affect products that end up in the mitochondria. These mutations can be inherited or spontaneous.
When the functioning of mitochondria is stopped then the cell is starved of energy. Therefore, depending on the type of cell, symptoms may vary. In general, cells that require the largest amounts of energy like cardiac muscle cells and nerves, are affected the most by faulty mitochondria.
If in a disease, different symptoms are generated but due to the same mutation then it is referred to as genocopies. In converse, diseases that have the same symptoms but are caused due to mutations in different genes are known as phenocopies.
The mitochondrial disease symptoms would vary largely, and they consist of
- Loss of muscle coordination and weakness
- Vision or hearing problem
- Heart, liver, or kidney disease
- Disabilities in learning
- Gastrointestinal problems
- Neurological problems
Some of the other conditions are –
- Diabetes
- Alzheimer’s disease
- Parkinson’s disease
- Schizophrenia
- Chronic fatigue syndrome, etc.
- Huntington’s disease
- Bipolar disorder
That means any sort of irregularities that occur in the function of mitochondria affect the health of humans, but often, it is difficult to identify because symptoms vary from person to person. In some cases, disorders of the mitochondria can be quite severe, and it can cause an organ to fail.
Mitochondrial Damage Leading to Alzheimer
Alzheimer’s disease is a serious disease and is one of the major examples of a neurodegenerative disease being linked to mitochondrial dysfunction. Alzheimer’s is characterized by progressive degeneration of neurons in the cortex and hippocampus, the brain regions known for intellectual capacity and memory storage, respectively. This degeneration results in a range of symptoms including dementia, difficulty in problem-solving, and mood disturbances.
Previously, it was assumed that abnormal aggregation, or clumping, of a protein, called amyloid-beta (Aβ) was the cause of Alzheimer’s disease, as the brains of Alzheimer’s patients are riddled with damage caused by these accumulated proteins. Latest studies show that symptoms of Alzheimer’s occur well before this damage is visible. Aβ, which is a protein that is normally found in healthy cells, only becomes toxic when it becomes misfolded or abnormal in the cell. It is now thought that these free pre-aggregated abnormal pieces of Aβ protein are the disease trigger; that even before the protein clumps together, the disease has started.
What Kind of Role Does Reactive Oxygen Species Make During Mitochondrial Dysfunction?
Another link between mitochondrial dysfunction and neurodegenerative diseases is the overproduction of damaging molecules called reactive oxygen species (ROS). ROS are highly active oxygen derivatives that can cause extreme injury and stress to cell structures, ultimately leading to cell death.
While ROS are normally created by mitochondria during ATP production, the cell has protective enzymes to convert ROS to less harmful forms, thereby preventing extensive damage. In cases of high ROS production, however, the cell is unable to defend itself and accumulates these toxic species over time.
Overproduction of ROS in neurons is frequently observed in patients with Parkinson’s and Huntington’s disease, as well as ALS. Additionally, mutations in ROS-converting enzymes are commonly found in patients with familial ALS.
Conclusion
Along being known as the cell powerhouse, mitochondria are mightier as they are the only structure present in the cell other than the nucleus having DNA.
